Make terminal ui binaries work well everywhere

Here's some screenshots of an emulator tui program that was compiled on
Linux, then scp'd it to Windows, Mac, and FreeBSD.

https://justine.storage.googleapis.com/blinkenlights-cmdexe.png
https://justine.storage.googleapis.com/blinkenlights-imac.png
https://justine.storage.googleapis.com/blinkenlights-freebsd.png
https://justine.storage.googleapis.com/blinkenlights-lisp.png

How is this even possible that we have a nontrivial ui binary that just
works on Mac, Windows, Linux, and BSD? Surely a first ever achievement.

Fixed many bugs. Bootstrapped John McCarthy's metacircular evaluator on
bare metal in half the size of Altair BASIC (about 2.5kb) and ran it in
emulator for fun and profit.

libc/calls/writev.c

@@ -27,6 +27,11 @@
 /**
  * Writes data from multiple buffers.
  *
+ * Please note that it's not an error for a short write to happen. This
+ * can happen in the kernel if EINTR happens after some of the write has
+ * been committed. It can also happen if we need to polyfill this system
+ * call using write().
+ *
  * @return number of bytes actually handed off, or -1 w/ errno
  */
 ssize_t writev(int fd, const struct iovec *iov, int iovlen) {